1. Field of the Invention
The present invention relates generally to rotors for rotary wing aircraft.
2. Description of the Related Art
One type of rotor used for rotary wing aircraft is a single structural blade assembly made up of outer blades and internal spar caps. The rotor has a tip at either extreme end. Each side of the blade of the rotor has a leading edge, that is moved into the wind during rotation of the rotor and a trailing edge that lies opposite the leading edge. Prior rotors, such as the one shown in U.S. Pat. No. 6,024,325 encase two flex-beam spars within the rotor. Manipulation of the location and orientation of the spars alters the pitch of the rotor, thus permitting control of the aircraft""s lift.
Centrifugal force tends to move all rotating masses toward the plane of rotation because the plane of rotation represents the maximum distance that a mass can attain from the axis of rotation. When a symmetrical rotor blade is at zero pitch relative to the plane of rotation, every mass above the plane of rotation is matched by an equal mass on the lower side, so no pitch control forces are applied. However, when the rotor blade is pitched up, such as when the pilot pulls up on the collective, masses near the leading edge are moved above the plane of rotation and masses near the trailing edge are moved below the plane of rotation. Centrifugal force tends to urge both masses toward the plane of rotation, causing the blade to want to move toward zero pitch. Therefore, to minimize pitch control forces, all masses should be as close to the pitch change axis as possible.
U.S. Pat. No. 6,024,325 describes a rotor for rotary wing aircraft. That patent is incorporated herein by reference. The rotor described in the ""325 patent provides a tremendous improvement over prior art rotors. However, some improvements are desirable.
A prototype constructed in accordance with the ""325 patent flew well, but the collective forces were high. The large separation of the spar caps at the point of attachment to the blade increased collective control forces because centrifugal force urged the spar caps toward the plane of rotation. Also, the rotor blade of the rotor described in the ""325 patent is swept back moving the outboard tip weights rearward to eliminate compressive stresses in the trailing edge, thus, moving the center of gravity of the lip weights toward the structural axis of the rotor. This sweep increases collective control forces because it increases the average distance of the tip weights away from the pitch change axis as centrifugal force tends to push the weights toward the plane of rotation. The sweep results in an aerodynamic xe2x80x9carrowxe2x80x9d effect wherein the airstrearn on the swept tip tends to force the blade toward zero pitch.
The present invention provides improvements over the prior art.
An object of the invention is to provide an improved rotor having lower collective control forces while maintaining the advantages provided by prior art rotors. A number of features are used to achieve this. First, the spar caps of the spar become joined to one another at the same point where bonding begins between the blade and the spar. The tendency of blade to want to flatten out is minimized since centrifugal force acting on the spar caps tries to force the caps to stay in the plane of rotation and by placing the caps as close to the pitch axis as possible, reduces the moment arm and the moment resisting the spar from twisting. Tip weights are located at or near the pitch change axis as well. In a preferred embodiment, the tip weights are located evenly in front of and behind the structural center of the inboard section of the spar. The blade of the rotor and the tip are not swept back.